1. Field of the Invention
This invention relates generally to a method and apparatus for exciting a light source, and more particularly to a method and apparatus for exciting a high intensity electrodeless light bulb.
2. Description of Related Art
An electrodeless lamp technology is evolving which is based on generating light that confines light emitting plasma generated and sustained by RF excitation. RF energy is coupled into an electrodeless lamp through a coupling coil or other means external to a bulb in order to form and sustain a plasma in the materials filling the bulb. The bulb normally contains an inert gas, such as argon, and an element from Group VI-A of the periodic table of elements, such as sulfur.
When such a bulb is excited, for example, by a single coil, energy is coupled into the bulb by transformer action as illustrated in FIG. 1. The primary winding of the transformer comprises an exciting coil 10 surrounding the bulb 12. The secondary winding comprises a single turn secondary contained within the exciting coil 10 and consists of a closed circular path or torus 14 of plasma formed in the bulb 12 when the contents are excited by an RF source 16.
The equivalent circuit of such a configuration is shown in FIG. 2. There reference numeral 18 denotes the transformer formed by the exciting coil 10 and torus 14. The electrical resistance of the torus is designated by reference numeral 20. An excitation signal V.sub.coil is generated by the RF source 16 and is coupled across the exciting coil 10. A load voltage V.sub.torus is depicted by the dashed closed circular line in FIG. 1 and comprises a voltage which appears across the secondary winding 14, i.e. the torus. When the exciting voltage V.sub.coil is applied to the coil 10, an H-field, which is normal to the plane of the exciting coil 10, is created within the area enclosed by the coil. Since the H-field is time varying due to the fact that the RF signal from the source 16 is an AC signal, it is accompanied by a toroidal E-field distribution in which the planes of the E-fields within the exciting coil parallel to the plane of the coil. The induced voltage around any enclosed E-field path is equal to the voltage per turn in the exciting coil times the ratio of the area enclosed by the toroidal path to the area enclosed by the exciting coil, that is, the voltage ratio is proportional to magnetic flux linkage.
Accordingly, as ionization occurs due to electromagnetic field heating of the material within the bulb, current flows in a closed toroidal path within the bulb, producing light by the heating of the material within the bulb. In any conducting toroidal path, the magnitude of current flow will equal the voltage induced around the closed path divided by the effective impedance of the path. The effective diameter of the resulting current path is such as to enclose the maximum cross section area of magnetic field, but is limited by cooling caused by the bulb's surface, which limits ionization near the inner surface of the bulb.
As the torus heats further, the Group VI-A element(s) are brought into the plasma, producing very bright light emission from a diffused region of the bulb. The spectrum of this light, which is dependent upon the fill, can be made to be very nearly that of sunlight.
Current flow in the torus counters much of the induced magnetic field within the torus. This causes the toroidal diameter to shrink. Stability is reached within the bulb when the voltage drop V.sub.torus across the torus due to coupling area falls to a sufficiently low level to minimize further build-up of toroidal current and further collapse of the torus due to cancelled magnetic field. This is the operating equilibrium condition of the lamp. If the equilibrium is not reached because of low resistance around the torus and current continues to rise, then the plasma will pinch off and the light will be extinguished.
Physically rotating the bulb creates a centrifugal force on the fill molecules which counteracts the constricting magnetic forces by pushing the molecules outward, tending to keep the torus as large as possible. The result is that the conducting toroid plasma emits more light and is less prone to extinguish due to ionic pinch off. Since this centrifugal force involves mechanical rotation of the bulb at speeds up to 10,000 Hz, it is inherently less than desirable for a commercial lamp system.